fixed bug 169718 with ancestors in patterns similar to same bug fixed in

* libxslt/pattern.c: fixed bug 169718 with ancestors in patterns
  similar to same bug fixed in libxml2
* tests/general/bug-160.*, tests/general/Makefile.am,
  tests/docs/bug-160.*, tests/docs/Makefile.am: added test provided
  by Aaron Kaplan
Daniel